1. Field of the Invention
This invention relates to a process for producing N-substituted maleimides. More particularly, it relates to a process for producing N-substituted maleimides by heating maleic anhydride and an aromatic or aliphatic amine, or maleic acid monoamides obtained from them, in an organic solvent in the presence of a catalyst.
N-Substituted maleimides are compounds which can be used in a very wide field of applications as raw materials or intermediates for pharmaceuticals, agricultural chemicals, dyes and high molecular substances.
2. Description of the Prior Art
Various processes for producing N-substituted maleimides have hitherto been known.
For example, there is known a method which comprises obtaining N-substituted maleimides by heating at 180.degree. C. maleic acid monoamides (maleamic acids), which are easily obtainable from maleic anhydride and amines such as butylamine, octylamine, decylamine and dodecylamine, to effect cyclodehydration. (L. E. Coleman et al., J. Org. Chem., 24, 135 (1959)). However, this method cannot be used in practice since it gives a low yield of intended products of only 15 to 50% and moreover yields a large amount of polymers of a polyimide structure as a by-product.
A well known laboratory method of preparation is to treat maleic anhydride and aniline in the presence of sodium acetate catalyst using a dehydrating agent such as acetic anhydride. (Org. Synth. Coll. Vol. 5, 944 (1973)). Though the method can give N-substituted maleimides in a relatively high yield (75 to 80%), it has a defect of high production cost since it requires the use of a stoichiometric amount of acetic anhydride, which results in additional cost of the auxiliary material, and thus it is unsuitable for industrial production.
On the other hand, a conceivably advantageous process for industrial production is to effect the cyclodehydration of maleic acid monoamides under milder conditions using an effective dehydration catalyst without using dehydrating agents.
Various attempts have been made also regarding to such a process. There have been proposed, for example, a method which uses a basic catalyst such as an alkali metal acetate, sodium hydroxide or triethylamine as a catalyst (Japanese Patent Application publication No. 24024/1972, corresponding to Canad. Pat. No. 906494 and West Germany Pat. No. 2100800) and a method which uses an acidic catalyst such as sulfuric acid and a sulfonic acid (Brit. Pat. Specification No. 1041027).
However, these methods using such catalysts are not yet fully satisfactory in suppressing side reactions since they give polymeric products as a by-product. Moreover, they require complicated steps in the separation and recovery of the catalyst and the removal of by-products from the reaction products and thus cannot be said to be an advantageous process for industrial production.
As described above, the prior art cyclodehydration of maleic acid monoamides in the presence of a catalyst gives relatively a large amount of side reaction products including polymeric by-products and consequently has drawbacks in yield, product purity and operation procedures. Thus, the suppression of side reactions including polymer formation has become an important problem to be solved.